Do you already a domain? If you do the registrar of your domain is where you'd go about adding the A Record for the ftp site. As for the step by step I can't really give you that as I don't know how your registrar or domain host works.
If you only have one external IP you don't need to assign it to the site, the port forward takes care of that. Also if you have a dynamic IP you'll need to use another service like, dyndns.